In semiconductor integrated circuit packaging, the process of electrically coupling a length of wire to a bond pad of a semiconductor die mounted on a lead frame to a lead of the lead frame is commonly known as wire bonding. For each connection between die and lead frame two bonds are required. A ball bond bonds one end of a length of wire to the die, and a stitch bond bonds the length of wire to a lead of the lead frame. Conventionally, the ball bond entails forming a ball on one end of a spool of wire, typically gold wire, prior to bonding the ball to a bond pad on a semiconductor integrated circuit using non-thermo compression, thermo compression or ultrasonic thermo compression. After the ball bond, a measured length of wire is provided from the spool of wire before the stitch bond is formed on a lead of the lead frame. The stitch bond also severs the length of wire from the spool, thereby freeing the end of the spool of wire on which a ball for the next connection is subsequently formed.
Bonding wire to the die and the lead frame is accomplished using a bond head having a capillary with a hardened tip, through which the wire from the spool is threaded. The bond head moves vertically along the z-axis above the die-lead frame assembly. The die-lead frame assembly is mounted on an x-y table which moves along the x and y axis to align the bonding locations on the die-lead frame assembly below the bond head. The movement of the bond head and the x-y table are co-ordinated and controlled manually or by a computer program during the bonding process.
Conventionally, the ball on the end of the spool of wire is formed by placing what is commonly known as an electrical flame-off (EFO) wand a predetermined distance from the end of the spool of wire. An electrical arc is then emitted between the end of the spool of wire and the EFO wand, forming a ball on the end of the spool of bonding wire. By varying the intensity and the duration of the electrical arc, the size of the ball that is formed can be adjusted to specific dimensions. To apply the electrical arc, a high voltage circuit generates a large potential difference between two terminals. One terminal is coupled to the; spool of wire, such that the wire forms an electrode, and the other terminal is connected to the EFO wand forming another electrode.
A conventional EFO wand is a hockey stick shaped stainless steel rod, the lower portion of which is positioned between the bond head and the die-lead frame assembly when forming a ball. Due to the size of the lower portion of the conventional EFO wand, and consequently the large clearance required between the bond head and the die-lead frame assembly, several disadvantages occur. A first disadvantage is the bond head must travel a long distance to the die which takes a significant period of time. Although the duration of one bond may not be significant, it should be noted that integrated circuits today have pin outs numbering in the hundreds. Hence, when viewing the duration of one bond from this perspective it becomes apparent that a minor change in the duration for one bond can have a considerable impact on the productivity of the wire bond process. A second disadvantage is the wide process variation in ball formation which affects the reliability of the ball bonds that are subsequently formed. A third disadvantage is the economic impact of the relatively shod life and high cost of replacement of the conventional EFO wand.
Hence, there exists a need for an EFO wand that is more economical to use in the wire bonding process, reduces process variation and improves the productivity of the wire bonding process.